IgG antibodies are large molecules of about 150kilodalton made of four peptide chains. It contains two identical class γ heavy chains of about 50 kDa and two identical light chains of about 25 kDa, thus a tetrameric quaternary structure. The two heavy chains are linked to each other and to a light chain each by disulfide bonds. The resulting tetramer has two identical halves, which together form the Y-like shape. Each end of the fork contains an identicalantigen binding site. The various regions and domains of a typical IgG are depicted in the figure to the left. The Fc regions of IgGs bear a highly conserved N-glycosylation site. The N-glycans attached to this site are predominantly core-fucosylated diantennary structures of the complex type. In addition, small amounts of these N-glycans also bear bisecting GlcNAc and α-2,6-linked sialic acid residues.
The various regions and domains of a typical IgG
There are four IgG subclasses (IgG1, 2, 3, and 4) in humans, named in order of their abundance in serum (IgG1 being the most abundant).
|Name||Percent||Crosses placenta easily||Complement activator||Binds to Fc receptor on phagocytic cells||Half Life|
|IgG1||66%||yes (1.47)*||second-highest||high affinity||21 days|
|IgG2||23%||no (0.8)*||third-highest||extremely low affinity||21 days|
|IgG3||7%||yes (1.17)*||highest||high affinity||7 days|
|IgG4||4%||yes (1.15)*||no||intermediate affinity||21 days|
|* Quota cord/maternity concentrations blood. Based on data from a Japanese study on 228 mothers.|